The invention relates to a process for producing conductive coatings, in particular strip conductors, and substrates selected from glass and enamelled steel, coated by this process. A screen-printable aluminum paste based on aluminum powder, at least one glass frit and an organic medium is used. The invention also relates to coated substrates obtainable by this process.
Sheets of glass and ceramic substrates for electronic uses are for various purposes often provided partially or over the entire surface with a conductive layer, the coating containing a metal as the component imparting conductivity and a glass composition as binder. Such coatings are often based on precious metals such as, for example, silver in the case of strip conductors on glass or gold and/or platinum metals in the case of strip conductors or electrodes on ceramic substrates, such as semiconductor ceramics.
The production of these strip conductors comprises the application of a metal powder and a liquid or paste-like composition containing glass frit and a subsequent firing at a firing temperature appropriately selected for the substrate.
Whereas the precious metals can be replaced partially by aluminum in the production of conductive coatings on ceramic substrates, it has not hitherto been possible to produce high-quality aluminum-based strip conductors on sheets of glass. Known compositions containing solely aluminum powder as conductive component require a heating temperature which is too high for glass and/or result in a coating having inadequate adhesion and/or inadequate conductivity.
U.S. Pat. No. 3,833,348 describes a process for bonding two parts made of silicon nitride. A composition to be used for this is based on a glass frit and aluminum. The preferred glass frits based on SiO2, Al2O3 and MnO have a melting point which is too high for application to glass.
U.S. Pat. No. 5,562,972 discloses a conductive paste for the production of moisture-resistant electrodes having a favorable ohmic contact between the electrode and a semiconductor element. The paste contains aluminum powder and a glass frit based on barium borosilicate or/and calcium borosilicate in a quantity of 5 to 40 wt. %, based on aluminum. Again, these pastes cannot be applied to glass, owing to their high melting point.
The composition containing aluminum powder and glass frit according to U.S. Pat. No. 5,856,015 does in fact contain a glass frit having a dilatometric softening point below 600xc2x0 C., but it is a glaze for a ceramic substrate. The aluminum content of this glazexe2x80x9430 wt. % at mostxe2x80x94is too low for use as conductive paste if an adequate conductivity is to be ensured.
According to U.S. Pat. No. 4,039,721, thick-layer strip conductors on a ceramic substrate are obtained by applying a paste consisting of 3 to 28 wt. % silver powder, 30 to 61 wt. % aluminum powder, 24 to 44 wt. % borosilicate glass frit to the substrate by means of screen printing and heating for a period of 0.2 to 1 hour at 850 to 105xc2x0 C. Although a xe2x80x9cglassy substratexe2x80x9d is referred to in the claims, only ceramic substrates are used in the Examples. The firing temperature does not favor the use of the paste on sheets of glass. It is necessary to use silver in addition to aluminum in order to achieve an adequate conductivity.
U.S. Pat. No. 5,358,666 discloses an ohmic electrode material consisting of 10 to 50 wt. % glass frit selected from zinc borosilicate and lead-zinc borosilicate and of a conductive component containing 48 to 96 wt. % aluminum and 4 to 52 wt. % silicon. These components, in the form of a paste in an organic medium, are applied to a ceramic substrate and heated at 600 to 800xc2x0 C. In order to ensure that the specific resistance is sufficiently stable even after storage in a moist atmosphere, it is imperative that the paste contain silicon. This document is not concerned with strip conductors on glass.
An object of the present invention is to provide a process whereby, with the use of a paste containing glass frit and metal powder, conductive coatings can be produced on substrates such as glass and enamelled steel, for example, strip conductors on glass for automobiles and the like. The paste to be used should preferably be silver-free and, after application to glass, capable of being heated under conventional firing conditions, in particular in the rapid firing conventional for sheets of glass for automobiles, and give rise to firmly adhering conductive layers.
A further object is the provision of a glass substrate, in particular a sheet of glass, supplied with a conductive coating and obtainable according to the invention, the coating being characterized by a specific resistance of less than 100 xcexcohmxc2x7cm and preferably less than 50 xcexcohmxc2x7cm, at the same time with good adhesion.
The present invention provides a process for producing a conductive coating, in particular strip conductors, on a substrate selected from glass and enamelled steel, comprising applying to the substrate a layer comprising a metal powder, at least one glass frit and a paste containing a liquid or thermoplastic medium and heating of the coating at a temperature in the range of 500 to 750xc2x0 C., which is characterized in that a screen-printable aluminum paste is used which contains (i) aluminum powder having a d50 value in the range of 1 to 10 xcexcm in a quantity of 40 to 80 wt. %, (ii) one or more glass frits having an initial softening temperature in the range of 400 to 700xc2x0 C. and a d50 value in the range of 1 to 10 xcexcm in a quantity of 5 to 40 wt. %, (iii) a liquid or thermoplastic medium consisting of one or more polymers and/or solvents in a total quantity of 10 to 35 wt. %, (iv) sintering aids in pulverulent form in a quantity of 0 to 10 wt. % and (v) 0 to 40 wt. % silver powder.
The aluminum paste can be applied in the form of thin layers to the substrate by means of screen printing or other method known in the practice of decorating. The term xe2x80x9cscreen-printablexe2x80x9d includes direct printing onto the substrate and the transfer technique.
Surprisingly, the aluminum pastes to be used according to the invention can be heated onto glass as substrate without difficulty in an atmosphere of air, even in a rapid firing, with firmly adhering, conductive layers based on aluminum, having a specific resistance of less than 100 xcexcohmxc2x7cm, being obtained. Depending on the glass frit used in the paste, layers having a specific resistance of less than 70 xcexcohmxc2x7cm are also obtainable, and particularly preferred pastes result in a specific resistance in the range of 20 to 50 xcexcohmxc2x7cm. Surprisingly, the heated conductive layers are also distinguished by having a high adhesive strengthxe2x80x94the scratch resistance is above 20 N, as determined by means of an Erichsen pin, which is drawn across the strip conductor produced according to the invention on glass. This scratch resistance is substantially preserved even after storage for 5 days at 70xc2x0 C. and 90% relative humidity. These positive properties can be achieved in the aluminum paste not only with lead-containing glass frits, but also with lead-free glass frits, with even better values surprisingly being obtainable by using zinc-containing glass frits.
Neither was it foreseeablexe2x80x94and this is a particular advantagexe2x80x94that, with the use of preferred aluminum pastes containing a zinc borosilicate glass frit, the specific resistance remains substantially constant within a range of about xc2x120xc2x0 C. below or above the conventional firing temperature.
The aluminum pastes contain preferably at least 50 wt. %, in particular 50 to 75 wt. % Al powder. The Al powder used is preferably granular and in particular spherical. Commercially available Al powder in the form of flakes is less suitable owing to the large diameter of the flakes (d90 equal to/greater than 30 xcexcm). The d50 value is suitably in the range of 1 to 10 xcexcm and in particular from 1 to 8 xcexcm. The d10 value is to be greater than 0.2 xcexcm, in particular greater than 0.5 xcexcm and particularly preferably in the range of 0.5 to 2 xcexcm, preferably below 15 xcexcm. The specific surface of the Al powder is conventionally in the range of 0.5 to 5 m2/g, but in individual cases these limits need not be met or can be exceeded. As the specific surface of the Al powder increases, however, the Al powder is less easily made into a paste, and this may possibly necessitate a decrease in the Al content. Particularly preferred Al powders are spherical, have a d50 value of around 5 xcexcm (xc2x12 xcexcm), a d90 value of below 15 xcexcm and a d10 value of above 1 xcexcm and a specific surface in the range of 0.5 to 3 m2/g. A portion of the Al powder may also be replaced by silver powder, but substantially silver-free pastes are preferred.
The glass frits used in the Al paste, both in connection with the quantity introduced and in connection with the chemical composition, greatly influence the properties of the heated Al coating. An important feature of the glass frit is the initial softening temperature. This can be determined with the aid of a test piece in a heating microscope. The initial softening temperature of glass frits used according to the invention is preferably in the range of 400 to 700xc2x0 C., in particular 400 to 650xc2x0 C. and particularly preferably from 420 to 580xc2x0 C.
The glass frits are used in the grain sizes conventionally employed for screen printing. Generally the d50 value is in the range of 0.5 to 10 xcexcm, preferably in the range of 1 to 5 xcexcm. It is advisable that the d90 value be less than 15 xcexcm, in particular less than 10 xcexcm and that the devalue be greater than 0.2 xcexcm, preferably greater than 0.5 xcexcm.
The quantity of glass frit introduced into the Al paste can be in the range of 5 to 40 wt. %, but a range of 5 to 30 wt. % is preferred and 9 to 25 wt. % is particularly preferred. The optimal quantity introduced as regards the properties attainable in a heated Al coating in turn depends largely on the composition of the glass frit. Zinc borosilicate frits, even at a particularly low frit content of 10 to 15 wt. %, produce an extremely low specific resistance accompanied by excellent adhesion.
Lead borosilicate frits and lead-free glass frits can be used. Among the lead-free glass frits, zinc-containing, bismuth-containing or zinc- and bismuth-containing glass frits are suitable, as well as glass frits based on SiO2, B2O3, TiO2 or Al2O3 and K2O. Glass frits which are primarily borosilicate frits and which also soften in the required temperature range are known to the experts. Representative frit compositions are based on the following obligatory components:
EP 0 790 220 B (in mol. %): SiO2 30-55, B2O3 10-25, TiO2 15-30, K2O 10-17;
EP 0 728 710 B (in mol. %): SiO2 40-50, B2O3 8-14, ZnO 13-19, TiO2 4-7, Na2O 10-15, K2O 0.1-2, F 1-5, Al2O3 0.1-3;
EP 0 267 154 B (in mol. %): SiO2 45-60, B2O3 6-13, ZnO 8-25, Na2O 5-14;
EP 0 558 942 (in mol. %): SiO2 10-44, B2O3 11-35, ZnO 31-50, Na2O 11-25;
EP 0 854 120 A (in wt. %): SiO2 10-25, B2O3 2-20, ZnO 3-15, Bi2O3 20-55, Na2O 1-10;
EP 0 803 480 A (in wt. %): SiO2 10-25, B2O3 20-40, ZnO 10-50, Bi2O3 0-15, Na2O 7-10; and
U.S. Pat. No. 5,714,420 (in wt. %): SiO2 20-35, B2O3 5-15, ZnO 5-45, Bi2O3 10-50, Na2O.
Particularly suitable zinc borosilicate frits (see Example 3) for the production of strip conductors on glass with extremely low specific resistance and very good adhesion have the following compositions (in wt. %): SiO2 10-25, B2O3 20-40, ZnO 10-50, Bi2O3 0-15, Na2O 7-15, Al2O3 3-8, F 0-2 and optical oxides such as CaO, TiO2 and ZrO2.
In another embodiment, the Al pastes may contain in addition pulverulent sintering aids in a quantity of up to 10 wt. %, preferably up to 6 wt. %. By sintering aids are meant all substances which lead to a higher degree of sintering at a given sintering temperature. Examples of sintering aids are metals, such as Zn, Mg, B, Si, in particular zinc and magnesium, fluorides, such as cryolite (AlF3.3 NaF), NaF, MgF2 and carbon. The action of the sintering aid may lie in the lowering of the melting point and/or in its reducing action during the firing process.
The organic medium serves to disperse the Al powder, the glass frit(s) and, if present, the sintering aid(s). The medium can be liquid or thermoplastic, and hence become liquid only at elevated temperature. Liquid organic media containing one or more polymers (=binders) and/or one or more solvents are preferred.
In principle, the medium may also be aqueous, but it is then advisable, in order to avoid a reaction between aluminum and water, that it contain in addition at least one compound from among the polybasic acids, such as maleic acid or its anhydride, succinic acid, polyacrylic acid, boric acid, sodium dihydrogen phosphate, sodium metasilicate or sodium triphosphate.
The medium particularly preferably contains both a binder and at least one organic solvent. The binder content is so chosen that a non-slip film is obtained after the Al paste has been dried. A quantity of binder in the range of 0.5 to 10 wt. % is particularly preferred, in particular 1 to 5 wt. %, based on the aluminum paste.
The selection of the binders is less crucial, provided that under the firing conditions they decompose and/or undergo combustion and in the course of this completely volatilise. Suitable binders are, for example, cellulose ether, acrylic esters and methacrylic esters, natural resins, colophony resins and modified alkyd resins.
The organic solvents used as components of the medium are those which volatilise during the firing free from bubbles and, if present, can dissolve the binder, and which lead to an aluminum paste having a suitable processing viscosity. Examples are terpene alcohols and terpene hydrocarbon; glycols and diglycols as well as ethers and esters of these; cyclic and branched hydrocarbons, such as isoparaffins having a boiling point in the range of 160 to 220xc2x0 C.; alcohols, ethers and esters having a boiling point in the range of 70 to 250xc2x0 C., in particular 100 to 220xc2x0 C. The quantity of solvent used is determined by the required viscosity and is in the range of 10 to 35 wt. %, preferably in the range of 10 to 30 wt. %. The Al paste particularly preferably contains 15 to 25 wt. % of a medium consisting of one polymer and one or more solvents. The Al paste to be used according to the invention can be prepared in the manner conventional for ceramic printing pastes, namely by intensive mixing of the components, for example, in a three-roll mill, disperser or in a ball mill.
The substrates are preferably sheets of glass, in particular those for motor vehicles. Other substrates are enamelled steel, xe2x80x9csteelxe2x80x9d meaning steel of any composition. The firing is carried out at a temperature in the range particularly preferably of 550 to 700xc2x0 C., in particular 590 to 690xc2x0 C. The Al paste is heated onto sheets of glass for automobiles under conditions approximately identical to those conventional for glass colorants, that is, in a rapid firing at 640 to 690xc2x0 C. for a period of 1 to 10 minutes, in particular 4 to 6 minutes.
The aluminum paste according to the invention is applied by means of conventional processes of the type known for the production of decorations on glass or ceramic. These are conventional direct and indirect printing processes, in particular screen-printing processes. Application by spraying, dipping or by means of other techniques for applying decoration is also possible.
The invention also provides a coated substrate, which is characterized in that the substrate is glass, the coating contains 60 to 90 wt. % aluminum and the specific resistance of the conductive layer is in the range of 10 to 70 xcexcohmxc2x7cm.
As already explained above, it is also possible to produce coated substrates of a type which have a higher conductivity, corresponding to a lower specific resistance, and nevertheless excellent adhesion. Particularly preferred glass substrates coated with an aluminum coating are characterized by a specific resistance in the range of 20 to 50 xcexcohmxc2x7cm and a scratch resistance of at least 20 N.
The coating on glass according to the invention can serve as an alternative to conventional strip conductors for antennae and plate heating and thus replace the currently conventional silver strip conductors. The problem of discoloration known in Ag coatings and caused by Ag migration is thereby also avoided. A coating on enamelled steel according to the invention can also be used as radiant panel heating.